Cleaning devices for transfer drums in electrophotographic or other electrostatic apparatus are well known. For example, color copiers presently available attach a sheet of paper to a transfer drum. The drum is rotated a plurality of times bringing a transfer surface of the sheet into transfer relationship with an electrophotographic imaging member. With each presentation of the transfer surface to the imaging member a different colored toner image is transferred to the transfer surface, generally under the urging of an electric field. Several color images are superposed in this manner forming a multicolor image on the transfer surface.
Inevitably toner and paper fibers get on the external cylindrical surface of the transfer drum itself where they then transfer to the rear of the next transfer sheet. Fur brush cleaners and web cleaners have been used commercially to clean the external cylindrical surface of transfer drums in such apparatus; see, for example, U.S. Pat. No. 3,819,263. Such cleaning devices are articulated from a position out of contact with the drum, when the apparatus is in a transfer mode, to a position in cleaning engagement with the drums, when the apparatus is in a cleaning mode. Fur brushes generally require vacuum or other fairly complex mechanism to get rid of the toner and also must be driven by a motor. Web cleaners are less expensive but the web itself must be indexed and the web must be changed periodically.
Transfer drums are also known which receive toner images directly to their external cylindrical surface without the interposition of a transfer sheet. In color apparatus, these drums receive more than one image in registration to their surface forming a multicolor image on the drum surface. The multicolor image is transferred to a transfer sheet either in the same nip as the original toner images were transferred or at a location remote from the original nip. Because of incomplete transfer to the transfer sheet the external cylindrical surface of these drums generally must be cleaned between formation of each multicolor image.
Roller cleaners have been used commercially to clean electrophotographic drums; see for example, U.S. Pat. Nos. 3,655,373; 3,807,853; 3,838,472; 4,101,215; and 4,530,596. In its simpliest form, a soft conductive rubber roller is allowed to roll with the drum, a bias is applied to the roller to attract the toner to it from the drum, and the softness of the rubber roller compared to the surface of the drum cooperates with that bias to clean the drum. The roller itself passes into contact with a scraping blade which scrapes the toner into a disposing mechanism, for example, an auger or the like.
Cleaning rollers have also been used for transfer drums in monocolor apparatus which rollers do not have to be articulated; see, for example, U.S. Pat. Nos. 4,183,655; 4,439,462; and 4,588,279. In U.S. Pat. No. 4,183,655 the toner is removed from a transfer drum to a roller under a first electrical field and then transferred back to the drum under an opposite direction electrical field for eventual transfer to the photoconductor and removal by the primary cleaning station. In this device, the roller does not actually engage the drum, but cleans across a small air gap.
A problem which must be overcome in cleaning transfer drums is that the drum is generally electrically biased to a high potential relative to the imaging member backing (usually ground) to attract toner in the transfer mode. If a similarly strong field is to be employed to move the same toner to a cleaning roller, a very high bias must be applied to the roller or expensive high voltage switching applied to the drum.